Ethnobotanicals for Storage Pest Management: Effect of powdered leaves of Olax zeylanica in suppressing infestations of rice weevil Sitophilus oryzae (Coleoptera: Curculionidae)

Abstract

The rice weevil, Sitophilus oryzae is one of the major pests of stored rice in Sri Lanka. This study investigates the effectiveness of the botanical Olax zeylanica in controlling infestations of the rice weevil.In two separate bioassays, contact/feeding and fumigant toxicity of powdered leaves of O. zeylanica were tested against 1-7 days old adult S. oryzae under laboratory conditions. Contact/feeding toxicity was tested by directly exposing adult weevils to 1.0, 3.0, 5.0 and 7.5g of leaf powder mixed with 100g of rice grains while fumigant toxicity was evaluated by using the same doses where theadults were exposed to fumes emitted from the leaf powders. In both bioassays 100% mortality of the adult S. oryzae was observed within 18 hours of exposure to 3.0, 5.0 and 7.5g doses of leaf powder. Percentage adult weevil mortality in treated rice tested with three doses of O. zeylanica leaf powder at all the time intervals (except for 1.0g) was significantly higher (p < 0.05) than that of the corresponding control. No Contact/feeding toxicity was recorded when adult weevils were directly exposed to 1.0g leaf powder whereas only 14% adult weevil mortality was observed even after 24hours of exposure to fumes ofleafpowder. Results also revealed that mortality increased both with increasing dose and time of exposure. In both bioassays a 100% adult weevil mortality was obtained after18 hours of exposure to 3.0g leaf powder of O. zeylanica or to its fumes.Moreover, LD50 values of 2.55g and 2.08g for leaf powders obtained after 12 hours of exposure to adult S. oryzae in contact/feeding toxicity test and fumigation test respectively.The results indicatedthat leaf powder of O. zeylanica is more toxic to adults S. oryzae when they were in direct contact with it.Findings of this study bears out the exceptionally high efficacy of O. zeylanica leaves applied directly mixed with the rice grains or introduced as a fumigant to suppress weevil infestations in stored grains and strengthen the possibility of using powdered leaves of O. zeylanica as an alternative to synthetic chemicals in storage insect pest management.